Well done for getting out between Christmas and NY- so easy to sink into a sea of sloth and not emerge. We're on holiday atm. Cant really run as nowhere to go but have been to yoga a few times.
Re ultras, women in particular, seem to gain more endurance as they age, and this helps the conversion to longer distances- many top trail runners are 40+ There is anecdotal evidence that having children improves women's ability to run longer distances but no-one really knows why (hormonal or biomechanics or just want to get away from the DC for long periods to time 
). It has certainly been true for me, but only after my second child.
ultras tend to be off road and most involve some unrunnable sections, necessitating a run-walk strategy, which makes them more sustainable than a straight run. The pacing is much slower and more varied- the course record for a 43km night trail race I do is 4.47 and that's by a professional female trail runner. My first 50km race this year will take close to 6 hrs, the second will take closer to 8 hrs (much tougher course), so you just settle into it and try to cover the ground as efficiently as possible.
The reason I do them is that relative to other people, I'm faster than over a shorter distance, so in a 20km trail race I don't place very well because I don't have the pace required, but I could keep going at a slower pace for longer than many of the people who would beat me over the shorter distance. My aim for 2016 is to do the Hong Kong 100km race. My gut feeling is that that will be too long, but I'll give it a go.